metamorphic
Gneiss (probable banded metamorphic rock)
Banded gneiss; typically quartz–feldspar–mica assemblage
AI-generated identification · may be incorrectMedium- to coarse-grained, gray with pale banding and dark mica-rich layers; vitreous to dull luster, interlocking crystals, no obvious cleavage as a whole. Individual minerals are generally Mohs 2–7; specific gravity about 2.6–2.8.
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Physical properties
Medium- to coarse-grained, gray with pale banding and dark mica-rich layers; vitreous to dull luster, interlocking crystals, no obvious cleavage as a whole. Individual minerals are generally Mohs 2–7; specific gravity about 2.6–2.8.
Formation & geological history
Formed when granite or other high-grade parent rock underwent intense heat and pressure, segregating minerals into compositional bands; commonly Precambrian to younger, depending on locality.
Uses & applications
Used as crushed aggregate, road stone, building stone, and occasionally decorative facing or landscaping material; usually low commercial value as a hand specimen.
Geological facts
The alternating light and dark bands are characteristic of gneissic foliation and reflect mineral segregation during metamorphism.
Field identification & locations
Identify it by distinct, irregular light/dark banding and visible quartz, feldspar, and dark mica; unlike slate, it is coarse-grained and does not split into thin sheets. Common in ancient continental shields and metamorphic mountain belts.
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